1. Field of the Invention
The present invention relates to a device for the automatic measurement without contact of the volume of a layer deposited on a substrate. It applies to deposits formed for electronic circuits by hybrid microelectronic technologies.
With the device of the invention, the equivalent of a volume can be measured; we could just as well measure a thickness, we would then have much richer information than the measurement of the thickness alone. The "thickness" measured could be called in this case "topological thickness".
2. Description of the Prior Art
In a general way it is known that a hybrid circuit for example is formed from an electrically insulated substrate on which are deposited different layers of electricity conducting, linearly resistive or not, dielectric or insulating materials.
If we take the case of resistances obtained from resistive inks deposited on a substrate, the thickness of the deposit needs to be accurately controlled for it determines the final value of the resistance according to the conventional equation: EQU R=.rho.(L/S) Equation (1)
R: resistance PA1 .rho.: resistivity of the material PA1 L: length of the resistance PA1 s: section of the resistance PA1 l: width of the resistance PA1 e: thickness of the resistance.
If we consider that the resistance is rectangular in section, we may write: EQU s=1.times.e
with
We may therefore write, supposing L/1=N (number of squares) EQU (1/R)=e/.rho.N Equation (2)
Since .rho. and N are constants, it can be seen that the value of the resistance is directly related to that of the thickness: 1/R=f (e).
Up to now, the technician corrects his process by measuring (e). Now at least three basic problems arise from this way of operating:
1. It is necessary to measure the thickness as soon as the deposit has been formed, i.e. according to the terms used in the technique, to measure in the "wet" or "raw" state, so as not to wait for the end of the procedure (about 60 to 90 minutes later) and risk throwing away a large number of the substrates because they are outside the values.
2. The technician knows that the term thickness is extremely vague and canot be more imprecise. In fact, the shape of the deposit is not a parallelepiped and only Equation (1) is applicable since it only considers the section of the resistive element without making assumptions as to the shape of this section. Now, all the manufacturers of silk-screen printed hybrid circuits use the Equation (2) because the thickness is the only measurement which they may apprehend. It can be seen in FIG. 1 that this is an illusion and that no fine correlation can be obtained between this measurement of the layer thickness "e" (FIG. 1) and the value of the resistance obtained after baking and that consequently silk-screen printing systems cannot be built which would be self correcting and which would thus be really automatic. Depending on the widths of the resistances, and their length, different shapes in section may be obtained; these result from the balance of the surface tension forces and the forces of gravity. Consider what information can the measurement of "e" (illustrated at the left of FIG. 1) give in the case of a resistance, e.g. in the case of resistance 2 of small width l1 (.ltoreq.1 mm), or that of a resistance 3, or that of resistance 4 for which l3 (seen in FIG. 1) is approximately greater than 2 mm and is greater than l2.
3. The methods for measuring the thickness used at present do not even allow the section, or average, thickness to be measured.
The solutions known at present are not adapted to the problem of measuring the thicknesses of silk-screen printed deposits or deposits made by another method. The different methods of measurement at present available may be presented by showing their limits, which will more clearly bring out the advantage of the present invention.
Measurement by a fluorescence RX, though it is effectively without contact, is however related to the nature of the atoms forming the layer. Now in so far as resistive inks are concerned, from the low values (a few ohms per square) to very high values (a few tens of megohms per square), their composition is essentially variable. Finally, the measurement takes between 5 and 30 seconds.
Measurement by back-scattering of beta rays comes up against the same difficulty as stated before since it depends directly on the composition of the material deposited. Generally, a difference of 20% in the atomic numbers of the atoms forming the substrate and the deposit is furthermore required; this prohibits measurements on the dielectric and insulating material.
Measurement by Eddy currents requires either non conducting deposits on non magnetic metals or conducting deposits on less conducting substrates. These limits prohibit it generally for use within hybrid microelectrics, principally when it is desired to make a wet measurement which is the purpose of the servo-control of the silk-screen printing machines.
Magnetic induction only relates to non conducting deposits on a magnetic substrate, so it is then in general not useable.
Ellipsometry and spectrophotometry, generally used in the measurement of the thickness of oxides and nitrides, are not at all adapted to the types of deposits used in hybrid microelectronics.
The methods based on Hall's effect, measurement of the microresistance or coulometric measurement are adapted to metals but not to ceramic materials and moreover they are not compatible with "wet" state measurement.
Profilometers are widely used in hybrid microelectronics but, on the one hand, they do not allow wet measurement, and on the other hand the measurement is very localised, since only a section is available in a given plane and not the whole of the volume. Finally, they are slow and difficult to integrate in an automatic measurement chain.
Optical section microscopes are also widely used in hybrid microelectronics as they provide contactless measurement, but they present numerous drawbacks. They require optical focusing which complicates the automation and, on the other hand, their field is very limited and it is impossible by construction to display the whole of a section even for a width as small as 1 mm.